This invention is related to passive component technology for fiber optics telecommunication application. The purpose of the invention is to improve the present technology related to the Interleaver function. Improvements concern issues related to manufacturing easiness, thermal stability, chromatic dispersion compensation and fine-ajustmentof the interleaver response.
With the growing of the telecommunication needs, network designers look toward systems with always higher transmission rates. In the temporal domain, clock period is increased to 40 Gbit per second. In the spectral domain, Wavelength Division Multiplexing (WDM) becomes a Dense Wavelength Division Multiplexing (DWDM) with standard channel spacing of 100 GHz and a short-term evolution to 50 GHz and 25 GHz channel spacing.
This very narrow channel spacing requires the development of new technologies in order to manage the signal of each channel. Interleaver is one of the responses. This device can actually translate a X GHz channel spacing line into two 2X GHz channel spacing lines. It becomes then easier to manage two lines with large channel spacing instead of one line with narrow channel spacing. An output interleaver can then recombine all channels into one single DWDM line as shown in FIG. 1.
It is desirable for an interleaver to have the following characteristics: Low Insertion Loss, low crosstalk, and low polarization sensitivity (Polarization Dependent Loss and Polarization Mode Dispersion). Also, it is desirable for the interleaver device to be useful over a large frequency bandwidth and temperature range. This last point introduces the notion of dispersion effect and thermal sensitivity, when a Fabry Perot cavity is used, for example, the desired spectral response for interleaver function could be achieved. But this response will be suitable only over a couple of degree temperature range and a narrow bandwidth and may not be useful over larger temperature and bandwidth ranges. Therefore, to be useful, thermal stability and chromatic dispersion compensation need to be either further controlled or compensated.
Finally, spectral interleaver response has to match the ITU (International Telecommunications Union) grid. With the thermal and chromatic dispersion effect, it is very desirable for any interleaver deployed to operate so that the wavelength components passing through the interleaver would conform to the ITU grid over the expected temperature and bandwidth ranges experienced by the interleaver.
State of the art schemes employ mainly two ways for thermal stabilization. Compensation can be achieved by using different materials with opposite thermal behavior or simply by using a heater to maintain constant the temperature of the device. The first technique requests a very high accuracy on the material length, so that their thermal effects entirely cancel one another. The second method requires external equipment for heater control and power supply, which is undesirable.
It is therefore desirable to provide an improved interleaver design where the above described difficulties are avoided.
Conforming to a predetermined wavelength grid of a communication protocol such as the ITU imposes another constraint on interleaver design in addition to thermal stability and chromatic dispersion compensation. One aspect of the invention is based on the observation that, if three birefringent devices are employed where the devices comprise three different optical materials, it is possible to construct an interleaver that controls the routing of the different wavelength components of the radiation in a manner that does not change over a predetermined range of temperatures or over a predetermined range of wavelengths, and at the same time conforms to a predetermined wavelength grid of a communication protocol. Preferably, the three birefringent devices pass radiation from two beams so that the first group of wavelength components of each of the two beams has a polarization state that is different from the polarization state of a second group of wavelength components in the two beams.
In order to avoid having to be very accurate in the dimensions of the optical lengths of the birefringent devices, preferably the birefringence of at least one of the birefringent devices is adjustable so that the effects of the three devices can be fine tuned to achieve the goal of temperature stability and chromatic dispersion compensation. In the preferred embodiment, one of the three birefringent devices is a Faraday rotator placed in a magnetic field. By changing the angle between the magnetic field and the optical path(s) of the optical beam or beams through the Faraday rotator, the behavior of the rotator can be fine tuned, so that the interleaver can achieve temperature stability, and/or chromatic dispersion compensation and preferably also achieve conformation to a predetermined wavelength grid of the communication protocol, without having to design the devices such that their optical path lengths are very accurate.
While the above-described aspects of the invention are particularly useful in an interleaver for interleaving wavelength components conforming to a predetermined wavelength grid of a communication protocol, certain aspects of the invention described above can also be used for other applications.
The present invention is an improvement of previous designs to reach a thermal stability and chromatic dispersion compensation. And it includes a way to finely adjust the interleaver spectral response to the ITU grid. One of the motivations is the feasibility of the device in the production line. This method presents the advantage of a possible adjustment of the interleaver spectral response after assembly, alignment and curing of the optical parts of the component.